Fluid separating device

ABSTRACT

The invention relates to a fluid separating device with a lower section with a fluid feeding device and a liquid discharging device, an upper section with a fluid feeding device and a gas discharging device, a contact device which is constructed in such a manner that gas, which rises from the lower section into the upper section, comes into contact with liquid which sinks from the upper section into the lower section. Thereby, the rising gases can be depleted of components which are soluble in said liquid. Furthermore, a measuring device for determining the quantity of liquid and/or the alterations thereof is provided.

FIELD OF THE INVENTION

The invention relates to a fluid separating device for separating gasesand liquids in several fluid flows and for separating liquid-solublecomponents from the gases. The device comprises a lower section with afluid feeding device and a liquid discharging device, an upper sectionwith a fluid feeding device and a gas discharging device, a contactdevice which is constructed in such a manner that gas which rises fromthe lower section to the upper section comes into contact with liquidwhich sinks from the upper section into the lower section, and ameasuring device for determining the quantity of liquid and/oralterations of the quantity of liquid in the lower section.

The fluid separating device according to the invention is particularlysuited for controlling the fluid flows in fuel cell systems, inparticular in DMFC systems.

PRIOR ART

In some fuel cells, instead of a pure fuel component, diluted fuel isused which will be designated below as fuel mixture, even if not allcomponents of this mixture are oxidisable substances.

In a direct methanol fuel cell (DMFC), the fuel mixture on the anodeside consists, for example, of methanol dissolved in water, the firstbeing the actual fuel. The water of this fuel mixture does not appear inthe net accounting equation of the cell reaction, as distinguished fromthe water arising as reaction product (on the cathode side), which hasto be discharged from the cell as the reaction product carbon dioxidearising on the anode side. That is, the anode fluid undergoes adepletion of methanol and an enrichment of CO₂ on its path from theanode inlet to the anode outlet. In order to be able to optimallyutilize the depleted anode fluid, too, and to avoid liquid losses, as arule, a circuit flow is provided on the anode side, wherein the anodefluid is again enriched with fuel (corresponding to its consumption)after it has left the anode outlet and fed to the anode inlet again. Inthe process, however, carbon dioxide has to be discharged from thecircuit flow.

In the DMFC system, the major proportion of carbon dioxide is present ina gaseous form as the solubility limit of carbon dioxide in the fuelmixture is quickly exceeded. (As water is quantitatively the dominantsubstance in the fuel mixture, the solubility limit of carbon dioxide inthe fuel mixture approximately corresponds to that of carbon dioxide inwater.) That is, the fluid exiting at the anode outlet is as a rule nohomogenous phase but a gas/liquid mixture. Due to the flow conditions,however, the liquid and the gaseous phases are not physically strictlyseparated from one another; gas bubbles are rather formed in the liquid.

In the DMFC system, the liquid phase exiting at the anode outlet is awater/methanol solution, as a rule saturated with CO₂; in the gaseousphase, CO₂ enriched with water vapour and methanol vapour is dominant.Thus, in an unregulated waste gas removal, the fuel (here: methanol)present in the gaseous phase would be thus lost for the system, which isunacceptable not only for economical, but also for health and safetyreasons. Furthermore, water in the form of water vapour would be lost,so that for maintaining the operation conditions, an external watersupply would be necessary, which is unacceptable with respect to thepractical use of the fuel cell.

The above-mentioned problems have to be taken into consideration in theconception and operation of a DMFC system, which is conventionally doneas described below.

FIG. 1 is a view of a typical fuel cell system according to the presentinternal prior art. The DMFC fuel cell is schematically and only forsimplification divided into a cathode side K and an anode side A.(“Side” is not to be understood figuratively: in fact, a DMFC fuel cellconsists, as a rule, of a so-called stack with alternating anode andcathode areas).

The fluid on the cathode side comprises an oxidising substance, such asoxygen, which is supplied by means of a metering device 1 in the form ofnormal ambient air.

In the process, the non-usable substances of the air, such as nitrogen,but also water arising as reaction product and CO₂ diffusing from theanode to the cathode side, are discharged as fluid 21 at the outlet.

The fluid on the anode side further comprises, apart from methanol, thesubstances water and carbon dioxide, the latter having to be dischargedfrom the fuel cell as waste gas 15.

In the system shown in FIG. 1, two separate separating devices areprovided which separate gas from the fluid 11 discharged at the anodeoutlet for separating CO₂ or and, respectively, recover watercorresponding to the losses on the anode side from the fluid 21discharged at the cathode outlet.

The recovered water 14 or the water/methanol mixture 13, respectively,is again supplied to the anode inlet of the fuel cell in a reflux 12 (bymeans of a pump 2), methanol M being admixed from a storage tank T via ametering pump 3. The purified waste gases 15, 16 (CO₂, dried exhaustair) are discharged to the surroundings.

One of the essential problems is to keep the amount of water containedin the system as constant as possible, so that the necessity of aseparate water supply can be avoided. As water in the form of watervapour can be discharged with the substances to be discharged on theanode side as well as with the waste gases (“exhaust air”) on thecathode side, the latter not only comprising product water (to bedischarged quantitatively), but also water to be recycled to the anodeside which flows from the anode to the cathode side due to the “waterdrag” effect, the maintenance of a constant amount of water in the fuelcell is very elaborate.

DESCRIPTION OF THE INVENTION

In view of these problems, it is an object of the invention to provide afluid separating device for a plurality of various fluid flows. It isespecially an object to provide a fluid separating device enabling afacilitated control of the liquid supply and the removal andpurification of waste gases in a DMFC fuel cell.

These objects are achieved by the fluid separating device having thefeatures of claim 1. Advantageous further developments are listed in thesubclaims.

The fluid separating device according to the invention comprises a lowersection with a fluid feeding device and a liquid discharging device, anupper section with a fluid feeding device and a gas discharging device,a contact device which is constructed in such a manner that gas whichrises from the lower section to the upper section comes into contactwith liquid which sinks from the upper section to the lower section, anda measuring device for determining the quantity of liquid in the lowersection and/or for determining the alteration of the amount of liquid.

The operating process of the arrangement is as follows: Liquidsubstances fed to the upper section or condensing therein sink downwards(in a flow or as single drops) due to the gravity effect. Gases fed tothe upper section or arising therein rise upwards. In the contactdevice, the sinking liquid and the rising gases are brought intocontact, whereby components of the gases soluble in the liquidtransition into the liquid phase and are thus withdrawn from the gasstream.

The contact device is permeable to liquid, but it can at least slow downthe downward motion of the liquid, e.g. by absorbing a certain amount ofliquid. When the capacity for the absorption is exceeded, it permits aliquid penetration to the lower section. Thereby, the components removedfrom the gas stream can also be collected in a liquid reservoir in thelower section.

Gaseous substances fed to the upper section can be directly dischargedvia a gas discharging device from the upper section. If the suppliedfluid comprises liquid as well as gaseous proportions, the gravityeffect separates them physically. The gas stream penetrated from thelower to the upper section is discharged via the gas outlet (gasdischarging device) of the upper section.

The amount of liquid collected in the fluid separating device can bedetermined by means of the measuring device for determining the amountof liquid in the liquid reservoir of the lower section. If necessary,additional measuring devices can be provided for determining the amountof water present in the contact device. In practice, this amount,however, can be assumed to be either constant or negligible, so thatwith the one measuring device in the lower section the amount of liquidcan be sufficiently precisely determined. In many applications,especially for control methods, the absolute amount of liquid does nothave to be determined. For taking appropriate measures, it can rather besufficient to determine alterations in the amount of liquid.

In a preferred further development, the contact device of the fluidseparating device comprises a sponge-like and/or porous material whichis permeable to gas and liquids but can absorb and store a certainamount of liquid. Only when this amount of liquid is exceeded, liquiddroplets are formed at the bottom of the material and finally fall downdue to gravity.

The sponge-like and/or porous material can also occupy nearly thecomplete lower section. In this case, the sponge or the porous materialitself forms the liquid reservoir.

In addition or as an alternative to the above-described furtherdevelopments, in another preferred further development, the contactdevice can comprise at least one bottom opening and at least oneoverflow pipe. The overflow pipe preferably extends downwards far intothe lower section, so that it is ensured that the lower opening of theoverflow pipe is situated below the liquid level of the lower sectionand gas does not penetrate from the lower section to the upper sectionvia the overflow pipe but exclusively via the at least one bottomopening. The operating conditions have to be adjusted such that thepressure in the lower section is higher than in the upper section.

In addition or as an alternative to the total amount of liquid, theproportion of a component can also be an important core value. In theintended purpose described in the introduction, this is in particularthe methanol proportion in the fuel mixture. Therefore, the fluidseparating device preferably comprises a measuring device fordetermining the amount and/or concentration of at least one liquidcomponent.

If the determined concentration deviates from the desired one, themissing proportion can be added by metered addition at an appropriatesite of the system. A direct metered addition into the liquid reservoirof the fluid separating device is particularly advantageous, so that ina preferred embodiment of the fluid separating device, a liquid feedingdevice ending in the lower section is provided.

In another preferred further development, in the upper section, thefluid separating device comprises means for condensing at least a partof gaseous components from the supplied fluid and/or means forevaporating at least a part of liquid components of the supplied fluid.

The first are mainly desired if no adequate condensation takes placebefore the supply. As an alternative or in addition, a condensation canalso be effected already before the entry into the fluid separatingdevice, for example, by means of a heat exchanger or a condensationtrap. The evaporation devices can include a heating for increasing thegaseous proportion of water at the expense of the liquid one.

By these means, the amount of liquid which is supplied to the completedevice via the fluid feeding device can be controlled.

Preferably, the fluid separating device comprises in its upper sectionmeans for avoiding a removal of liquid via the gas discharging device ofthe upper section. These means can, for example, comprise gas-permeablemembranes in the gas discharging device. A suitably dimensioned,funnel-like means which prevents liquid from flowing from the lowerareas of the fluid separating device into the upper gas inlet or gasoutlet area, respectively, if the complete device tips over, is alsoadvantageous.

Below, the invention is illustrated with reference to two particularlypreferred embodiments.

In the drawings:

FIG. 1 shows the schematic structure of a DMFC-system (internal priorart).

FIG. 2 shows a first preferred embodiment of the invention;

FIG. 3 shows the schematic structure of a DMFC system using the firstpreferred embodiment of the fluid separating device according to theinvention;

FIG. 4 shows a second preferred embodiment of the invention;

FIG. 5 shows the schematic structure of a DMFC system using the secondpreferred embodiment of the fluid separating device according to theinvention.

FIG. 1 has already been described in the introduction. Modifications tothe arrangement shown in FIG. 1 are described with reference to FIGS. 3and 5 which show arrangements resulting from the use of preferredembodiments of the fluid separating device according to the invention.

FIG. 2 shows a first preferred embodiment of the fluid separating device100 according to the invention.

A lower section 110 comprises a fluid feeding device 111 and a liquiddischarging device 112. An upper section 120 comprises a fluid feedingdevice 121 and a gas discharging device 122. Via the two fluid supplydevices 111, 121, gases, liquids and gas/liquid mixtures can besupplied. The gas discharging device 122 is conveniently (but notnecessarily) provided at the upper side of the upper section 120. Thisdoes not absolutely have to be a tubular outlet 122. The complete uppercover surface (or a part thereof can be replaced by a gas-permeable butwaterproof (or at least hydrophobic) membrane, for example by a porousPTFE-foil.

The upper section 120 is separated from the lower section 110 by asponge-like contact device 130 which is designed such that a part ofliquid substances supplied to the upper section 120 via the fluidfeeding device 121 or condensing in the upper section 120, is absorbedby the contact device. Only when the absorption capacity of the spongeis exceeded, drops are released at its bottom surface and fall into theliquid reservoir of the lower section 110. Gaseous substances, however,can leave the upper section 120 via the gas discharging device 122.

The fluid feeding device 121 preferably ends in a gas room of the lowersection 110. To this end, it is provided at an upper area of the lowersection 110. Alternatively or additionally, it can comprise a flexibletube with a float, which are designed such that fluid fed via the fluidfeeding device 121 first enters the gas room of the lower section 110.

As in the upper section 120, in the lower section 110, too, the gravitycauses a gas/liquid mixture fed by the fluid feeding device 121 to beseparated into physically separated phases. The liquid is collected in aliquid reservoir of the lower section 110 and can be discharged via theliquid discharging device 112. Gaseous substances, however, can onlyescape from the lower section 110 to the upper section 120 and have topenetrate the contact device 130. In the process, components of the gasstreaming upwards can be dissolved in the slowed down or collectedliquid and fed to the liquid reservoir in the lower section 120 withreleased liquid drops. Thereby, methanol can be easily, but effectively,withdrawn from a waste gas mixture with methanol vapours and fed to theliquid reservoir situated at the bottom of the lower section 110. Thepurified waste gas can be discharged to the outside together with theexhaust air via the gas discharging device 122.

The alteration of the amount (or the amount itself) of the liquidcollected in the reservoir can be measured by a measuring device 140.The measurement can, for example, be performed capacitively by means oftwo capacitor plates. If the liquid is, for example, mainly water, itsdielectric constant is 80 times higher than the gaseous phase, so thatalterations of the amount of liquid can be very accurately determined bymeans of alterations of the capacity of the capacitor arrangement. If anappropriate calibration has been conducted, absolute values can also bedetermined.

Below, the functions of the first preferred embodiment of the inventionare illustrated with reference to FIG. 3 in the use of a DMFC system.

In comparison with FIG. 1, in FIG. 3 the same or comparable features areprovided with reference numerals increased by 100.

In DMFC fuel cells, due to an electrochemical reaction, gaseous CO₂ iscreated which has to be removed from the anode space of the fuel cells.In the gaseous phase, normally, however, there are also components ofthe fuel mixture, that means, for example, water vapour or methanoltransitioned into the gaseous phase. The proportions of this substancedepend on the respective vapour pressure, that is they are generallyincreased with temperature. In order to ensure a closed water circuitand avoid the discharge of fuel to the surroundings, measures have to betaken to separate these substances from the gaseous phase.

By diffusion or pulling effects (water drag), CO₂ and water, and alsolower quantities of methanol, can penetrate the cathode space.

Thus, at the anode outlet, a fluid is discharged which comprises aliquid as well as a gaseous phase. The liquid phase is a water/methanolmixture (with water being the dominant component), in which CO₂ isdissolved. The gaseous phase consists of CO₂, water vapour and methanolvapour.

At the cathode outlet, a fluid is discharged which comprises a gaseousphase and possibly also a liquid phase. The gaseous phase essentiallyconsists of oxygen-depleted air (exhaust air), water vapour, with loweramounts of CO₂. The liquid phase is essentially condensed water. Forachieving a closed water supply, water may be discharged to thesurroundings only in such quantities that arise as product water.

In the arrangement outlined in FIG. 3, the two separate separatingdevices of FIG. 1 are replaced by an embodiment of the fluid separatingdevice 100 according to the invention.

The fluid discharged at the cathode outlet is supplied to the fluidseparating device 100 via the fluid feeding device 121 of the uppersection 120. The fluid discharged at the anode outlet is supplied to thefluid separating device 100 via the fluid feeding device 111 of thelower section 110. In both fluids, first a physical separation into aliquid phase and a gaseous phase is effected due to the gravity effect.

The recovered water/methanol mixture is again supplied to the anodeinlet of the fuel cell via the liquid discharging device 112 (by meansof pump 2), and in the process, corresponding to the amount of spentmethanol, pure methanol M is admixed from a storage tank T by means of ametering pump 3. The purified waste gases (CO₂, exhaust air) aredischarged to the surroundings via the gas discharging device 122.

For maintaining the operation, it is necessary to keep the total amountof water in the system constant within certain tolerance limits, thatis, for example, to avoid an excessive (i.e. exceeding the waterproduction) discharge of water in connection with the waste gasdischarge, or vice-versa to increase the discharge in case of anincrease of the amount of water.

In the present example, alterations of the amount of water can betracked by means of alterations of the capacity of the measuring device140. A controlling device S can activate the metering device 1 on thebasis of these alterations in order to reduce the fluid flow on thecathode side, which effects a reduced water discharge from the system,or to increase it, which increases the water discharge. An alternativeor additional control mechanism is indicated in FIG. 5 and consists ofthe control of the system temperature (with higher temperatures, thehumidity of the gases and thus the water discharge are increased).

With the fluid separating device according to the invention, it istherefore comparably easy to fulfil the condition of a constant amountof water in the fuel cell.

FIG. 4 shows a second preferred embodiment of the fluid separatingdevice 200 according to the invention. In comparison with FIG. 2, thesame or comparable features are provided with reference numeralsincreased by 100.

The lower section 210 also comprises a fluid feeding device 211 (endingin the upper area of section 210) and a liquid discharging device 112.In addition, a liquid feeding device 213 (ending in the lower area ofsection 210) is provided. The upper section 220 comprises, as in theembodiment which is shown in FIG. 2, a fluid feeding device 221 viawhich the gases, liquids and gas/liquid mixtures can be fed, as well asa gas discharging device 222 (which is arranged at the top, but can alsobe arranged laterally). Due to the gravity effect and the greatlyreduced flow velocity and—if necessary, supported by a not showncondensing device—in the upper area of section 220, a physicalseparation of the gaseous and liquid phase proportions is effected,wherein the first can be discharged via the gas discharging device 222and the latter are conducted away downwards via a funnel-like draindevice 225. The funnel-shape is particularly convenient but notabsolutely necessary. By an appropriate selection of the length of thefunnel tube, it can be avoided that in case of a tipping of the wholedevice 200 liquid penetrates from the bottom to the top. Furthermore,the funnel tube can also have a contacting effect, as here liquid andgases are passing each other. This effect can even be amplified if asponge-like absorbent material is provided in the funnel tube (method ofoperation as described with reference to FIG. 2). The two sections 210,220 are separated by a tub-like contact device 230 comprising anoverflow pipe 231 ending in the lower section 231, so that a part ofliquid substances which are conducted downwards via the drain device 225is collected by the contact device 230 and can flow into the lowersection 210 only when a certain level is achieved (when the upper edgeof the overflow pipe 231 is exceeded).

Gaseous substances which enter the lower section 210 together with thefluid supplied via the fluid feeding device 211 can escape upwardsthrough a bore 232 in the contact device 230, but they have to passthrough the liquid collected therein. In the process, gas components,such as methanol, can be dissolved and supplied to the liquid in thelower section 220 via the overflow pipe.

With the embodiments of FIGS. 2 and 4, very effective waste gaspurification is possible, whereby the methanol content of the wastegases can be drastically reduced. The humidity content of the wastegases can also be greatly reduced. However, it should be kept in mindthat an amount of water corresponding to the arising amount of water hasto be discharged, such that the amount of water in the system does notcontinually increase. Therefore, the devices 100 and 200 should bedimensioned with respect to the range of application such thatapproximately this amount of product water is separated as water vapourwith the waste gases 122 or 222, respectively, which, however, willnormally be possible only approximately and not exactly. To be able todetermine deviations therefrom and to take countermeasures, measuringdevices 140, 240 for determining the amount of liquid or alterations ofthe amount of liquid are provided in the lower section.

Apart from the already mentioned examples, the countermeasure can alsoconsist in a heating which controls the ratio of gaseous to liquid waterin the fluid fed on the cathode side. Such a heating can be providedseparately of and outside the fluid separating device, but it can alsobe integrated into the fluid separating device. Alternatively, acontrollable capacitor or a heat exchanger where the discharged cathodefluid passes by can be used as a countermeasure.

In the embodiment which is shown in FIG. 4, a level meter 240 whichdetermines the level of the liquid surface is provided as a measuringdevice. As the liquid is electrically conductive due to the CO₂dissolved therein, the level metering can be effected by means of theconductivity: for example, electrode pairs which are short-circuited bythe liquid can be provided at different levels. Alternatively, thecapacities of capacitors or the alterations of the capacities can beused as measured quantity. Optical measuring methods which are based onthe different optical properties of the gaseous phase and the liquid arealso technically easy to realize; among these properties are: index ofrefraction, absorption, transmission. Thus, for example, diode pairsarranged in pairs can be provided one of which each serves astransmitter and the other one as receiver diode and by means of whichone can detect whether there is any liquid between them.

The level meter 240 is preferably to be arranged and designed such thatreasonable measuring results can be determined even if the orientationof the fluid separating device is not vertical. A more centralarrangement is clearly preferred to the outlined lateral attachment.

By means of the fuel consumption to be determined, for example, one candetermine how much fuel has to be added to the circuit flow by metering.In the present case (FIG. 4), the fuel M can be directly fed to thelower section 210 via the liquid feeding device 213, which enables afacilitated design of the anode circuit. As an alternative to the fuelconsumption, the amount of the fuel M to be added by metering can bedetermined by measuring the fuel concentration in the liquid in thelower section 210.

FIG. 5 serves for illustrating the mode of operation of the secondpreferred embodiment of the invention with reference to its use in aDMFC system. With respect to FIG. 3, the same or comparable features areprovided with reference numerals increased by 100.

As distinguished from FIG. 3, in this case, methanol is directlysupplied from the tank T to the water/methanol mixture in the lowersection of the fluid separating device 200.

The amount of pure methanol M to be added by metering can, for example,be determined by a (not shown) concentration sensor in the lower section210 or the methanol consumption which can be calculated by means of thesystem efficiency.

Alterations of the amount of water can be tracked by means of the levelsensor 240. A controlling device S can activate a heater H (for exampleprovided in the anode circuit) on the basis of these alterations tocorrespondingly adapt the water discharge from the system: at highertemperatures, the amount of water discharged with the waste gases isincreased.

In the arrangement which is shown in FIG. 5, the lower section of thefluid separating device 200 simultaneously serves as mixing chamber.

The above-described embodiments only serve for illustrating theprinciples underlying the invention. In particular, the fact that thesecond preferred embodiment (FIG. 4) of the invention comprisesadditional means with respect to the first one (FIG. 2), should not beconstrued as restricting. Of course, these additional means can also beintegrated into the first embodiment, and they can also be omitted inthe second embodiment. The scope of protection of the invention isexclusively defined by the following patent claims.

1. Fluid separating device, comprising: a lower section with a fluidfeeding device and a liquid discharging device, an upper section with afluid feeding device and a gas discharging device, a contact device,which is provided such that gas rising from the lower section into theupper section is contacted with liquid which sinks from the uppersection into the lower section, and a measuring device for determiningthe amount of liquid in the lower section and/or for determiningalterations of the amount of liquid.
 2. Fluid separating deviceaccording to claim 1, in which the contact device comprises asponge-like and/or porous material.
 3. Fluid separating device accordingto claim 2, in which the contact device essentially occupies thecomplete lower section.
 4. Fluid separating device according to claim 1,in which the contact device comprises at least one bottom opening and atleast one overflow pipe.
 5. Fluid separating device according to claim1, comprising a measuring device for determining the amount and/or theconcentration of at least one liquid component.
 6. Fluid separatingdevice according to claim 1, comprising a liquid feeding device endingin the lower section.
 7. Fluid separating device according to claim 1,in which the upper section comprises means for condensing at least apart of gaseous components of the supplied fluid and/or evaporating atleast a part of liquid components of the supplied fluid.
 8. Fluidseparating device according to claim 1, in which the upper sectioncomprises means for avoiding a discharge of liquid through the gasdischarging device of the upper section.
 9. Fluid separating deviceaccording to claim 2, in which the contact device comprises at least onebottom opening and at least one overflow pipe.
 10. Fluid separatingdevice according to claim 3, in which the contact device comprises atleast one bottom opening and at least one overflow pipe.
 11. Fluidseparating device according to claim 2, comprising a measuring devicefor determining the amount and/or the concentration of at least oneliquid component.
 12. Fluid separating device according to claim 3,comprising a measuring device for determining the amount and/or theconcentration of at least one liquid component.
 13. Fluid separatingdevice according to claim 4, comprising a measuring device fordetermining the amount and/or the concentration of at least one liquidcomponent.
 14. Fluid separating device according to claim 2, comprisinga liquid feeding device ending in the lower section.
 15. Fluidseparating device according to claim 3, comprising a liquid feedingdevice ending in the lower section.
 16. Fluid separating deviceaccording to claim 4, comprising a liquid feeding device ending in thelower section.
 17. Fluid separating device according to claim 5,comprising a liquid feeding device ending in the lower section. 18.Fluid separating device according to claim 2, in which the upper sectioncomprises means for condensing at least a part of gaseous components ofthe supplied fluid and/or evaporating at least a part of liquidcomponents of the supplied fluid.
 19. Fluid separating device accordingto claim 3, in which the upper section comprises means for condensing atleast a part of gaseous components of the supplied fluid and/orevaporating at least a part of liquid components of the supplied fluid.20. Fluid separating device according to claim 4, in which the uppersection comprises means for condensing at least a part of gaseouscomponents of the supplied fluid and/or evaporating at least a part ofliquid components of the supplied fluid.